Black snow is the unofficial term for the soot that inevitable falls after a nuclear strike. Because a nuclear strike vaporises everything in a short radius it is burned to ash and sent into the sky with the force of the blast. Due to most of the mater in the ash being completely destroyed it has a very low density and is carried by any wind for a decent range of a few dozen kilometers/miles. The ash then falls to the ground like a light snow(except black/dark gray). Because of the radioactivity of the bomb the black snow is highly radioactive and lethal, if you see it run to your nearest fallout shelter or basement.